Piezo sensor system for detecting the needle lift of a nozzle of a common rail injector

ABSTRACT

The invention relates to a nozzle needle  4  of an injection nozzle  1,  which is disposed within an injector housing and is biased against a housing shoulder  5  by way of a spring  2.  A piezo-element  7  is between the spring  2  and the housing shoulder  5,  to determine the nozzle needle path during opening and closing of the nozzle opening  3.  For this purpose, the piezo-element  7  has two electrical connectors  8.1, 8.2  in the region of the faces  7.1, 7.2,  respectively.

The invention relates to an injection nozzle for a common-rail injector,having a nozzle needle closing off a nozzle opening, biased by way of aspring, whereby the spring is disposed between a housing shoulder and acontact surface of the nozzle needle.

The injection amount and the start of injection are importantcharacteristics for the optimal operation of diesel engines. Determiningthem permits the load-dependent and rpm-dependent adjustment of theinjection process in a closed control circuit. The current methods fordetermining the needle stroke are imprecise, since the needle stroke isdetected at a distance from the nozzle opening, i.e. the nozzle needletip. The influence of a control piston, i.e. a connecting rod between asetting element and the nozzle needle falsifies the result.

An injection nozzle for fuel is already known from U.S. Pat. No.6,235,736 B1. The injection nozzle has a nozzle needle as well as acontrol piston disposed axially to the nozzle needle. The control pistonis biased upward via a first spring, and is set in the axial directionby way of a piezo-element. The nozzle needle is biased downward, inother words opposite to the control piston, by way of a second spring,and closes the nozzle opening in the bottommost position. By means of amovement of the control piston as the result of a setting movement ofthe piezo-element, the piston releases a flow-through opening for furl,so that the nozzle needle is moved out of its seat upward, counter tothe spring force. After a stroke (h), the nozzle needle hits against alower face of the control piston with its upper face, and therebygenerates an additional upward setting force. As soon as thepiezo-element is switched to be current-free, the control piston movesback into its upper end position and closes off the through-flowopening. The nozzle needle is then back in pressure force equilibrium,so that the resulting spring force performs the closing movement andcloses the nozzle opening once again. The position of the nozzle needleis detected by way of the position of the control piston.

The invention is based on the task of configuring and arranging aninjection nozzle in such a manner that optimal detection of the movementof the nozzle needle is guaranteed.

This task is accomplished, according to the invention, in that apiezo-element for detecting the spring force is provided between thehousing shoulder and the spring. This achieves the result that thespring force resulting from the spring bias and the nozzle needlestroke, and therefore the setting movement of the nozzle needle isdetermined in simple manner. The piezo-element can be disposed directlybehind the spring, so that no additional costs result from designchanges.

It is advantageous that a piezo-element is provided between the contactsurface of the nozzle needle and the spring. The resulting spring forceand therefore the setting movement of the nozzle needle is alsodetermined by means of the arrangement of the piezo-element between thenozzle needle and the spring.

For this purpose, it is advantageous that the piezo-element isconfigured to be ring-shaped or as a toroid having a first face and asecond face lying opposite the first face, and has a first electricalconnector in the region of the first face, and a second electricalconnector in the region of the second face. The piezo-element istherefore disposed between the force introduction points of the springand the housing shoulder or the contact surface, respectively, so thatthe load-dependent charge shift of the piezo-element is determined byway of the first and the second electrical connector.

Furthermore, it is advantageous that the setting path x of the nozzleneedle can be determined by way of the function $x = \frac{Q}{d_{p}D}$where Q represents the charge of the piezo-element, d_(p) represents thepiezoelectric coefficient, and D represents the spring stiffness of thespring.

By means of the arrangement of the piezo-element within the very thickwalls of the metal injection injector, any pick-up of magnetic andelectrical alternating fields acting on the measurement signal isgreatly attenuated.

For this purpose, it is also advantageous that the displacement chargecan be determined in simple manner, by means of integration of thedisplacement current of the piezo-element during a movement.

Finally, according to a preferred embodiment of the solution accordingto the invention, it is provided that the intermediate values for thesetting path x can be interpolated between two end positions of thesetting path x of the nozzle needle. Both the resulting spring force andthe piezo-element are linear with regard to the setting path x, i.e. thesetting force, and have a very low hysteresis effect.

The decrease in stiffness of the spring as the result of aging, i.e.fatigue of the piezo-element, can be corrected by taking the drift inthe measurement values into account.

The influence of the temperature results in an expansion of the nozzleneedle and of the injector housing, as well as a change in thepiezo-element characteristics. This temperature dependence is calculatedby way of the maximal stroke and taken into consideration accordingly.

The dead time between the turn-on control signal for the setting elementof the nozzle needle and the setting signal of the nozzle needle,according to the invention, which results from the mass inertia of thespring, can be used to uncouple the setting signal.

It is also advantageous that the housing shoulder and the piezo-elementhave a common opening disposed concentric to the piezo-element. In thisopening, a setting element for the nozzle needle, not shown, isdisposed.

Other advantages and details of the invention are explained in theclaims and the description, and are shown in the drawing:

In the drawing, a detail of an injection nozzle 1 within an injectorhousing, shown in part, having a housing shoulder 5 and a lower housingpart 1.1, is shown. In the housing part 1.1, a nozzle opening 3, whichis opened and closed by way of a nozzle needle 4, is provided. Thenozzle needle 4 has a contact surface 6 with which the nozzle needle 4rests against a spring 2. As a counter-bearing on the side of the spring2 opposite the nozzle needle 4, a piezo-element 7 is provided, which inturn rests against the housing shoulder 5. Thus the nozzle needle 4 isbiased relative to the piezo-element 7 and the housing shoulder 5, byway of its contact surface 6 with the spring 2. In this connection, thepiezo-element 7 is configured in cylindrical shape and has an averagediameter that corresponds to the average diameter of the spring 2.

The piezo-element 7 has a first face 7.1 that rests against the spring2, and a second face 7.2 that rests against the housing shoulder 5. Inthis connection, the first face 7.1 has a first electrical connector8.1, and the second face 7.2 has a second electrical connector 8.2. Thecharge displacement current I within the piezo-element 7 is captured byway of the electrical connectors 8.1, 8.2. The opening movement x thatproceeds from a closed, if applicable biased, position of the nozzleneedle 4 generates a growth in force acting on the piezo-element 7,which is proportional to the setting path x, whereby the proportionalityfactor corresponds to the spring constant D. The setting path x of thenozzle needle 4 can thereby be determined, in simple manner, by means ofthe formula $x = \frac{Q}{d_{p}D}$where Q corresponds to the charge shift within the piezo-element 7 thatoccurs as a result of the change in force, and d_(p) represents thepiezoelectric coefficient.

1. Injection nozzle (1) for a common-rail injector, having a nozzleneedle (4) closing off a nozzle opening (3) of the injection nozzle (1),biased by way of a spring (2), wherein the spring (2) is disposedbetween a housing shoulder (5) and a contact surface (6) of the nozzleneedle (4), and a piezo-element (7) is provided between the spring (2)and the housing shoulder (5), wherein the housing shoulder (5) and thepiezo-element (7) have a common opening (9), disposed concentric to thepiezo-element (7), which extends from the housing shoulder (5) to thecontact surface (6).
 2. Device, particularly as recited in claim 1,wherein the piezo-element (7) is configured to be ring-shaped or as atoroid having a first face (7.1) and a second face (7.2) lying oppositethe first face (7.1), and has a first electrical connector (8.1) in theregion of the first face (7.1), and a second electrical connector (8.2)in the region of the second face 7.2.
 3. Device as recited in claim 1,wherein the setting path x of the nozzle needle (4) can be determined byway of the function $x = \frac{Q}{d_{p}D}$ where Q represents the chargeof the piezo-element (7), d_(p) represents the piezoelectriccoefficient, and D represents the spring stiffness.
 4. Device as recitedin claim 1, wherein the displacement charge Q can be determined by meansof integration of the displacement current of the piezo-element (7)during a movement.
 5. Device as recited in claim 1, wherein theintermediate values for the setting path x can be interpolated betweentwo end positions of the setting path x of the nozzle needle (4). 6.Device as recited in claim 1, wherein a piezo-element (7) is providedbetween the spring (2) and the contact surface (6).